3. nitrogen cycle

3. NITROGEN CYCLE

SOIL 5813Soil-Plant Nutrient Cycling and

Environmental QualityDepartment of Plant and Soil Sciences

Oklahoma State UniversityStillwater, OK 74078

email: [email protected] Tel: (405) 744-6414

SOIL 5813Soil-Plant Nutrient Cycling and

Environmental QualityDepartment of Plant and Soil Sciences

Oklahoma State UniversityStillwater, OK 74078

email: [email protected] Tel: (405) 744-6414

ORGANICMATTER

MESQUITERHIZOBIUMALFALFASOYBEAN

BLUE-GREEN ALGAEAZOTOBACTERCLOSTRIDIUM

PLANT AND ANIMAL RESIDUES

R-NH2 + ENERGY + CO2

R-NH2 + H2O

R-OH + ENERGY + 2NH3

MATERIALS WITH NCONTENT < 1.5% (WHEAT STRAW)

MATERIALS WITH NCONTENT > 1.5%(COW MANURE)

MICROBIAL

DECOMPOSITION

HETEROTROPHICAMINIZATION

BACTERIA (pH>6.0)FUNGI (pH<6.0)

AMMONIFICATION

GLOBAL WARMING

pH>7.0

2NH4+ + 2OH-

FIXED ONEXCHANGE SITES

+O2

Nitr

osom

onas

2NO2- + H2O + 4H+

IMMOBILIZATION

NH3 AMMONIA -3NH4

+ AMMONIUM -3N2 DIATOMIC N 0N2O NITROUS OXIDE 1NO NITRIC OXIDE 2NO2

- NITRITE 3NO3

- NITRATE 5

OXIDATION STATES

ATMOSPHERE

N2ONON2

N2O2-

NH3

SYMBIOTIC NON-SYMBIOTIC

+ O2Nitrobacter

FERTILIZATION

LIGHTNING,RAINFALL

N2 FIXATION

DENITRIFICATION

PLANTLOSS

AMINOACIDS

NO3-

POOL

LEACHING

AMMONIAVOLATILIZATION

NITRIFICATION

NH2OH

Pseudomonas, Bacillus,Thiobacillus Denitrificans,and T. thioparus MINERALIZATION

+ NITRIFICATION

IMMOBILIZATION

NO2-

MICROBIAL/PLANT SINK

TEMP 50°F

pH 7.0

LEACHING LEACHING

DENITRIFICATIONLEACHING

LEACHINGVOLATILIZATIONNITRIFICATION ADDITIONS

LOSSES

OXIDATION REACTIONS

REDUCTION REACTIONS

HABER BOSCH

3H2 + N2 2NH3

(1200°C, 500 atm)

Joanne LaRuffaWade ThomasonShannon TaylorHeather Lees

Department of Plant and Soil SciencesOklahoma State University

INDUSTRIALFIXATION

Nitrogen cycle not well understood

Temperature and pH included

reduction/oxidation

tillage (zero vs. conventional)

C:N ratios (high, low lignin)

Fertilizer source and a number of other variables.

Mechanistic models would ultimately lead to many 'if-then' statements/decisions that could be used within a management strategy.

Denitrification Volatilization

Leaching Leaching

>50°F

<50°F

7.0soil pH

Assuming that we could speed up the nitrogen cycle what would you change?

1. Aerated environment (need for O2)

2. Supply of ammonium

3. Moisture

4. Temperature (30-35C or 86-95F) <10C or 50F

5. Soil pH

6. Addition of low C:N ratio materials (low lignin)

Is oxygen required for nitrification?

Does nitrification proceed during the growing cycle? (low C:N ratio)

Plants remove O2 to incorporate N into amino forms

aminoaminoaminoaminoacidsacidsacidsacidsNHNHNHNH 3333

nitrite reductasenitrite reductasenitrite reductasenitrite reductasenitrate reductasenitrate reductasenitrate reductasenitrate reductase

NONO 22NONO 33

N recommendations

1. Yield goal (2lb N/bu)

a. Applies fertilization risk on the farmer

b. Removes our inability to predict 'environment' (rainfall)

2. Soil test

a. For every 1 ppm NO3, N recommendation reduced by 2lbN/ac

3. Potential yield

Nitrite accumulation?

1. high pH

2. high NH4 levels (NH4 inhibits nitrobacter)

Inorganic Nitrogen Buffering

Ability of the soil plant system to control the amount of inorganic N accumulation in the rooting profile when N fertilization rates exceed that required for maximum yield.

So

il P

rofi

le In

org

an

ic N

A

cc

um

ula

tio

n, k

g/h

a

Gra

in y

ield

, kg

/ha

Annual Nitrogen Fertilizer Rate, kg/ha

0 40 80 120 160 200 240

Point where increasing applied N no longerincreases grain yield

500

400

300

200

100

0

Point where increasing applied N increases soilprofile inorganic N accumulation

Range (buffer) where increasingapplied N does not increasegrain yield, but also where noincrease in soil profile inorganic N is observed

Soil-Plant Inorganic N Buffering

4000

3000

2000

1000

0

FertilizerFertilizer

NHNH44, NO, NO33NHNH44, NO, NO33

Organic Matter PoolOrganic Matter PoolOrganic Matter PoolOrganic Matter Pool

InorganicInorganicNitrogenNitrogenInorganicInorganicNitrogenNitrogen

00

2222

4545

6767

9090

112112

00 100100 200200 300300 4004003030

6060

9090

120120

150150

180180

210210

240240

270270

300300

NONO33---N, kg ha-N, kg ha-1-1

Udic Argiustoll, 0-240 cm, #502

N Rate kg haN Rate kg ha-1-1

00

3434

6767

134134

269269

N Rate kg haN Rate kg ha-1-1 N Rate kg haN Rate kg ha-1-1

00 100100 200200 300300 4004003030

6060

9090

120120

150150

180180

210210

240240

270270

300300

NONO33---N, kg ha-N, kg ha-1-1

Udic Argiustoll, 0-300 cm, #505

De

pth

, cm

De

pth

, cm

De

pth

, cm

De

pth

, cm

If the N rate required to detect soil profile NO3 accumulation always exceeded that required for maximum yields, what biological mechanisms are present

that cause excess N applied to be lost via other pathways prior to leaching?

Nitrogen Buffering Mechanisms

1. Increased Applied N results in increased plant N loss (NH3)

Table 3. Forage, grain and straw N uptake and estimated plant N loss, experiments 222, 1996-1997, and 502, 1997

Location Fertilizer Applied Total N Uptake 1996 1997 N P K Forage Grain Straw Loss/

Gain Forage Grain Straw Loss/

Gain ----------kg ha-1 yr-1--------- -------------------------------------------kg N ha-1---------------------------------------- 222 0 29 38 29.40 23.47 12.74 -6.81 18.76 22.54 8.04 -11.82 45 29 38 38.59 32.10 18.54 -12.05 42.81 23.13 21.43 -1.75 90 29 38 70.72 40.63 27.50 2.59 96.62 31.01 55.02 -6.32 135 29 38 102.49 48.41 39.41 14.67 143.61 51.69 71.93 5.1 SED 8.20 4.40 2.79 19.91 2.90 11.91 N rate linear *** ** *** ** *** ** N rate quadratic ns ns ns ns ** ns 502 0 20 56 29.46 32.83 11.08 -14.45 23 20 56 56.21 50.01 26.68 -20.48 45 20 56 127.96 57.05 47.54 23.37 67 20 56 132.12 63.56 40.15 28.41 90 20 56 182.29 90.54 63.05 28.70 112 20 56 191.84 105.39 44.90 41.55 SED 24.79 14.65 9.55 N rate linear *** *** *** N rate quadratic ns ns * Loss/gain determined by subtracting forage N uptake at flowering from total N in the grain and straw at maturity. *, **, *** significant at the 0.05, 0.01, and 0.001 probability levels, respectively. SED = standard error of the difference between two equally replicated treatment means.

Lees, H.L., W.R. Raun and G.V. Johnson. 2000. Increased plant N loss with increasing nitrogen applied in winter wheat observed with 15N. J. Plant Nutr. 23:219-230.

photosynthesis carbohydrates

respiration

carbon skeletons

aminoacidsNH3

reducing power

nitritereductase

nitratereductase

ferredoxinsiroheme

NO 2NO 3

NADH or NADPH

Bidwell (1979), Plant Physiology, 2nd Ed.Metabolism associated with nitrate reduction

Francis, D.D., J.S. Schepers, and M.F. Vigil. 1993. Post-anthesis nitrogen loss from corn. Agron. J. 85:659-663.



2. Higher rates of applied N - increased volatilization losses




3. Higher rates of applied N - increased denitrification

Burford and Bremner (1975) found that denitrification losses increased under anaerobic conditions with increasing organic C in surface soils (0-15 cm) (wide range in pH & texture).

Denitrifying bacteria responsible for reduction of nitrate to gaseous forms of nitrogen are facultative anaerobes that have the ability to use both oxygen and nitrate (or nitrite) as hydrogen acceptors. If an oxidizable substrate is present, they can grow under anaerobic conditions in the presence of nitrate or under aerobic conditions in the presence of any suitable source of nitrogen

Burford and Bremner, 1975

Aulakh, Rennie and Paul, 1984





4. Higher rates of applied N - increased organic C, - increased organic N

0.040.040.040.04

0.050.050.050.05

0.060.060.060.06

0.070.070.070.07

0.080.080.080.08

0.090.090.090.09

0.10.10.10.1

0000 40404040 80808080 120120120120 160160160160 2002002002000.40.40.40.4

0.50.50.50.5

0.60.60.60.6

0.70.70.70.7

0.80.80.80.8

0.90.90.90.9

TSNTSNTSNTSN

OCOCOCOC

#406#406

To

tal

So

il N

, %

To

tal

So

il N

, %

To

tal

So

il N

, %

To

tal

So

il N

, %

Org

anic

Car

bo

n,

%O

rgan

ic C

arb

on

, %

Org

anic

Car

bo

n,

%O

rgan

ic C

arb

on

, %

N Rate, kg/haN Rate, kg/haN Rate, kg/haN Rate, kg/ha

SED TSN = 0.002SED TSN = 0.002SED TSN = 0.002SED TSN = 0.002

SED OC = 0.03SED OC = 0.03SED OC = 0.03SED OC = 0.03

Raun, W.R., G.V. Johnson, S.B. Phillips and R.L. Westerman. 1998. Effect of long-term nitrogen fertilization on soil organic C and total N in continuous wheat under conventional tillage in Oklahoma. Soil & Tillage Res. 47:323-330.






5. Increased applied N - increased grain protein

Gra

in N

up

take

, kg

/ha

Annual Nitrogen Fertilizer Rate, kg/ha

0 40 80 120 160 200 240

80

60

40

20

0

Point where increasing applied N no longerincreases grain yield

Increased grain N uptake (protein) at N rates in excess of that requiredfor maximum yield

Continued increase ingrain N uptake, beyond thepoint where increasingapplied N increases soilprofile inorganic Naccumulation

0 20 40 60 80 100 120 14020

30

40

50

60

70

80

# 222# 222

N rate, kg/ha

Gra

in N

Up

take

, kg

/ha

Y = 29.7 + 0.28x - 0.00055x2

r2=0.90

9.4 =19%






5. Increased applied N - increased grain protein

6. Increased applied N - increased forage N

7. Increased applied N - increased straw N

VolatilizationVolatilizationVolatilizationVolatilization

DenitrificationDenitrificationDenitrificationDenitrification

LeachingLeaching

NHNH33, N, N22NHNH33, N, N22

NO3NO3

Microbial PoolMicrobial PoolMicrobial PoolMicrobial Pool

NHNH44NHNH44

NONO33NONO33

NONO22NONO22

7-80 kg N/ha/yr7-80 kg N/ha/yr7-80 kg N/ha/yr7-80 kg N/ha/yr

NONONONONN22OONN22OO

NN22NN22

15-40 kg N/ha/yr15-40 kg N/ha/yr15-40 kg N/ha/yr15-40 kg N/ha/yrNH3NH3

0-50 kg N/ha/yr0-50 kg N/ha/yr0-50 kg N/ha/yr0-50 kg N/ha/yr

UreaUreaUreaUrea

Organic ImmobilizationOrganic ImmobilizationOrganic ImmobilizationOrganic Immobilization10-50 kg N/ha/yr10-50 kg N/ha/yr10-50 kg N/ha/yr10-50 kg N/ha/yr

0-20 kg N/ha/yr0-20 kg N/ha/yr

Fertilizer N Fertilizer N Fertilizer N Fertilizer N

AppliedAppliedAppliedApplied

11

22

33

44

55

55

22

Olson and Swallow, 1984Olson and Swallow, 1984Sharpe et al., 1988Sharpe et al., 1988Timmons and Cruse, 1990Timmons and Cruse, 1990

Olson and Swallow, 1984Olson and Swallow, 1984Sharpe et al., 1988Sharpe et al., 1988Timmons and Cruse, 1990Timmons and Cruse, 1990

11

Mills et al., 1974Mills et al., 1974Mills et al., 1974Mills et al., 1974Matocha, 1976Matocha, 1976Matocha, 1976Matocha, 1976DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964DuPlessis and Kroontje, 1964Terman, 1979Terman, 1979Terman, 1979Terman, 1979Sharpe et al., 1988Sharpe et al., 1988Sharpe et al., 1988Sharpe et al., 1988

44

Aulackh et al., 1984Aulackh et al., 1984Colbourn et al., 1984Colbourn et al., 1984Bakken et al., 1987Bakken et al., 1987Prade and Trolldenier, 1990Prade and Trolldenier, 1990

Aulackh et al., 1984Aulackh et al., 1984Colbourn et al., 1984Colbourn et al., 1984Bakken et al., 1987Bakken et al., 1987Prade and Trolldenier, 1990Prade and Trolldenier, 1990

33

Francis et al., 1993Francis et al., 1993Hooker et al., 1980Hooker et al., 1980O’Deen, 1986, 1989O’Deen, 1986, 1989Daigger et al., 1976Daigger et al., 1976Parton et al., 1988Parton et al., 1988

Francis et al., 1993Francis et al., 1993Hooker et al., 1980Hooker et al., 1980O’Deen, 1986, 1989O’Deen, 1986, 1989Daigger et al., 1976Daigger et al., 1976Parton et al., 1988Parton et al., 1988

Chaney, 1989Chaney, 1989Sommerfeldt and Smith, 1973Sommerfeldt and Smith, 1973Macdonald et al., 1989Macdonald et al., 1989Kladivko, 1991Kladivko, 1991

Chaney, 1989Chaney, 1989Sommerfeldt and Smith, 1973Sommerfeldt and Smith, 1973Macdonald et al., 1989Macdonald et al., 1989Kladivko, 1991Kladivko, 1991

NHNH44+OH+OH-- NH NH33 + H + H22OONHNH44+OH+OH-- NH NH33 + H + H22OO

N Buffering MechanismsN Buffering Mechanisms

NHNH44 fixation (physical) fixation (physical)NHNH44 fixation (physical) fixation (physical)

Industrial view of the Nitrogen Cycle

Nutrient Overload: Unbalancing the Global Nitrogen Cycle

Carbon Cycle

NITROGEN Cycle LinksNITROGEN Cycle Links

Urea

1. Urea is the most important solid fertilizer in the world today.

2. In the early 1960's, ammonium sulfate was the primary N product in world trade (Bock and Kissel, 1988).

3. The majority of all urea production in the U.S. takes place in Louisiana, Alaska and Oklahoma.

4. Since 1968, direct application of anhydrous ammonia has ranged from 37 to 40% of total N use (Bock and Kissel, 1988)

5. Urea: high analysis, safety, economy of production, transport and distribution make it a leader in world N trade.

6. In 1978, developed countries accounted for 44% of the world N market (Bock and Kissel, 1988).

7. By 1987, developed countries accounted for less than 33%

Koch Industries7.5 million metric tons of N fertilizer/year

WorldTotal Production N, P, and K216 million metric tons

Share of world N consumption by product group

1970 1986 2004

Ammonium sulfate 8 5 2

Ammonium nitrate 27 15 14Urea 9 37 50

Ammonium phosphates 1 5Other N products (NH3) 36 29 30Other complex N products 16 8

Urea Hydrolysisincrease pH (less H+ ions in soil solution) urease enzyme required

CO(NH2)2 + H+ + 2H2O --------> 2NH4+ + HCO3

-

pH 6.5 to 8

HCO3- + H+ ---> CO2 + H2O (added H lost from soil solution)

bicarbonate

CO(NH2)2 + 2H+ + 2H2O --------> 2NH4+ + H2CO3 (carbonic acid)

pH <6.3

H2CO3 CO2 + H2Ocarbonic acid

During hydrolysis, soil pH can increase to >7 because the reaction requires H+ from the soil system.

(How many moles of H+ are consumed for each mole of urea hydrolyzed?) 2

In alkaline soils less H+ is initially needed to drive urea hydrolysis on a soil already having low H+.

In an alkaline soil, removing more H+(from a soil solution already low in H+), can increase pH even higher

NH4+ + OH- ---> NH4OH ---->NH3 + H2O

pH = -log[H+]

Calculate pH of 2.0x10-3M solution of HCl

HCl is completely ionized so

[H+] = 2.0 x 10-3M

pH = -log(2.0x10-3)

= 3 – log 2.0

= 3 - 0.30

= 2.70

◦ pH = pKa + log [(base)/(acid)]◦ pKw = pH + pOH◦ 14.00 = pH + pOH

◦ At a pH of 9.3 (pKa 9.3) 50% NH4 and 50% NH3◦ pH Base (NH3) Acid (NH4)◦ 7.3 1 99◦ 8.3 10 90◦ 9.3 50 50◦ 10.3 90 10◦ 11.3 99 1

Chemicals A and B react to form C and D

A + B = C + D

Equilibrium Constant (K) K = [C][D] / [A][B]

6

7

8

9

10

0 20 40 60 80 100

pH

%

NH 3

4+NH

Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH

for a dilute solution.

H20 H+ + OH-

As the pH increases from urea hydrolysis, negative charges become available for NH4

+ adsorption because of the release of H+ (Koelliker and Kissel)

Decrease NH3 loss with increasing CEC (Fenn and Kissel, 1976)

Assuming that pH and CEC are positively correlated, what is happening?

Relationship of pH and BI (?) none

In acid soils, the exchange of NH4+ is for H+ on the exchange

complex (release of H here, resists change in pH, e.g. going up)

In alkaline soils with high CEC, NH4 exchanges for Ca, precipitation of CaCO3 (CO3

= from HCO3- above) and one H+ released which helps

resist the increase in pH

However, pH was already high,

pH

CEC** on soils where organic matter dominates the contribution to CEC then there should be a positive relationship of pH and CEC.

5

6

7

8

9

0 2 4 6 8 10 12 14 16 18 20

SOIL MIX 3-High Buffering Capacity

SOIL MIX 2-Moderate Buffering Capacity

SOIL MIX 1-Low Buffering Capacity

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16 18 20

3SO

IL S

URFA

CE p

H

DAYS AFTER APPLICATION

kg N

H -N

/ha

VOLA

TILI

ZED

Soil surface pH and cumulative NH3 loss as influenced by pH

buffering capacity (from Ferguson et al., 1984).

N Rate =

112 kg/ha

Ernst and Massey (1960) found increased NH3 volatilization when liming a silt loam soil. The effective CEC would have been increased by liming but the rise in soil pH decreased the soils ability to supply H+

Rapid urea hydrolysis: greater potential for NH3 loss. Why?

Management:

•dry soil surface

•Incorporate

•localized placement- slows urea hydrolysis

H ion buffering capacity of the soil:Ferguson et al., 1984

(soils total acidity, comprised of exchangeable acidity + nonexchangeable titratable acidity)

A large component of a soils total acidity is that associated with the layer silicate sesquioxide complex (Al and Fe hydrous oxides). These sesquioxides carry a net positive charge and can hydrolyze to form H+ which resist an increase in pH upon an addition of a base.

H+ ion supply comes from:

1. OM

2. hydrolysis of water

3. Al and Fe hydrous oxides

4. high clay content (especially 2:1, reason CEC’s are higher in non-weathered clays is due to isomorphic substitution – pH independent charge)

Soil with an increased H+ buffering capacity will also show less NH3 loss when urea is applied without incorporation.

1. hydroxy Al-polymers added (carrying a net positive charge) to increase H+ buffering capacity.

2. strong acid cation exchange resins added (buffering capacity changed without affecting CEC, e.g. resin was saturated with H+).

resin: amorphous organic substances (plant secretions), soluble in organic solvents but not in water (used in plastics, inks)

Consider the following

1. H+ is required for urea hydrolysis2. Ability of a soil to supply H+ is related to amount of NH3 loss3. H+ is produced via nitrification (after urea is applied): acidity generated is not beneficial4. What could we apply with the urea to reduce NH3 loss?

an acid; strong electrolyte; dissociates to produce H+;increased H+ buffering; decrease pH

reduce NH3 loss by maintaining a low pH in the vicinity of the fertilizer granule (e.g. H3PO4)

Comment: Ferguson et al. (1984).

“When urea is applied to the soil surface, NH3 volatilization probably will not be economically serious unless the soil surface pH rises above 7.5”

UREASE inhibitors

“Agrotain” n-butyl thiophosphoric triamide

http://www.agrotain.com

Nitrosomonas inhibitors

“NSERVE” 2-CHLORO-6-(TRICHLOROMETHYL) PYRIDINE

http://jeq.scijournals.org/cgi/content/abstract/32/5/1764

Computation/commodity Production, mTWorld consumption of fertilizer-N 90,000,000

Fert-N used in cereals (60% of total applied)0.60 * 82,906,340 = 54,000,000

World Cereal Production, mT

Sorghum3%

Rye1%Oats

2%Millet1%

Barley8%

Rice28%

Corn29%

Wheat28%

NEED for INCREASED NUE

World grain N removal, 1996 %N mTWheat 2.13 12,502,267Corn 1.26 7,439,266Rice 1.23 7,007,101Barley 2.02 3,154,192Sorghum 1.92 1,356,807Millet 2.01 580,032Oats 1.93 596,012Rye 2.21 508,788Total N removed in cereals 33,144,465

N removed in cereals (from soil & rain, 50% of total) 16,572,232

NUE = ((N removed - N soil&rain)/total N applied) 33%

Savings/yr for each 1% increase in NUE 489,892 mT

Value of fertilizer savings $479/mT N $234,658,462

2005 >$400,000,000

____________________________________ World cereal grain NUE 33% Developed nation cereal NUE 42% Developing nation cereal NUE 29%

____________________________________ 1% increase in worldwide cereal NUE

= $234,658,462 fertilizer savings 20% increase in worldwide cereal NUE

1999 = $4.7 billion

2005, > 10 billion

Flowchart for NUE

http://www.nue.okstate.edu/NUE_etc.htm

Role of NH4 nutrition in Higher Yields (S.R. Olsen)

•Glutamine-major product formed in roots absorbing NH4

•NO3 has to be transported to the leaves to be reduced

•Wheat N uptake was increased 35% when supplying 25% of the N as NH4 compared to all N as NO3 (Wang and Below, 1992).

•High-yielding corn genotypes were unable to absorb NO3 during ear development, thus limiting yields otherwise increased by supplies of NH4 (Pan et al., 1984).

•Assimilation of NO3 requires the energy equivalent of 20 ATP/moleNO3, whereas NH4 assimilation requires only 5 ATP/mole NH4 (Salsac et al., 1987).

•This energy savings may lead to greater dry weight production for plants supplied solely with NH4 (Huffman, 1989).

photosynthesis carbohydrates

respiration

carbon skeletons

aminoacidsNH3

reducing power

nitritereductase

nitratereductase

ferredoxinsiroheme

NO 2NO 3

NADH or NADPH

Bidwell (1979), Plant Physiology, 2nd Ed.Metabolism associated with nitrate reduction

Discussion:Global Population and the Nitrogen Cycle

p.80 nitrous oxide

Increasing use of fertilizer N results in increased N2O. Reaction of nitrous oxide (N2O) with Oxygen contribute to the destruction of ozone.

Atmospheric lifetime of nitrous oxide is longer than a century, and every one of its molecules absorbs roughly 200 times more outgoing radiation than does a single carbon dioxide molecule.

“In just one lifetime, humanity has indeed developed a profound chemical dependence.”

Factors Affecting Soil Acidity

Acid: substance that tends to give up protons (H+) to some other substance

Base: accepts protonsAnion: negatively charged ionCation: positively charged ion

Base cation: ? (this has been taught in the past but is not correct)

Electrolyte: nonmetallic electric conductor in which current is carried by the movement of ions

H2SO4 (strong electrolyte)

CH3COOH (weak electrolyte)

H2O

HA --------------> H+ + A-

potential active

acidity acidity

1. Nitrogen Fertilization

A. ammoniacal sources of N

2. Decomposition of organic matter

OM ------> R-NH2 + CO2

CO2 + H2O --------> H2CO3 (carbonic acid)

H2CO3 ------> H+ + HCO3- (bicarbonate)

humus contains reactive carboxylic, phenolic groups that behave as weak acids which dissociate and release H+

3. Leaching of exchangeable bases/Removal

Ca, Mg, K and Na (out of the effective root zone)

-problem in sandy soils with low CEC

a. Replaced first by H and subsequently by Al (Al is one of the most abundant elements in soils. 7.1% by weight of earth's crust)

b. Al displaced from clay minerals, hydrolyzed to hydroxy aluminum complexes

c. Hydrolysis of monomeric forms liberate H+

d. Al(H2O)6+3 + H2O -----> Al(OH)(H2O)++ + H2O+

monomeric: a chemical compound that can undergo polymerization

polymerization: a chemical reaction in which two or more small molecules combine to form larger molecules that contain repeating structural units of the original molecules

4. Aluminosilicate clays

Presence of exchangeable Al

Al+3 + H2O -----> AlOH= + H+

5. Acid Rain

NITROGEN:

Key building block of protein molecule

Component of the protoplasm of plants animals and microorganisms

One of few soil nutrients lost by volatilization and leaching, thus requiring continued conservation and maintenance

Most frequently deficient nutrient in crop production

Nitrogen Ion/Molecule Oxidation States

Range of N oxidation states from -3 to +5.

oxidized: loses electrons, takes on a positive charge

reduced: gains electrons, takes on a negative charge

Illustrate oxidation states using common combinations of N with H and O

H can be assumed in the +1 oxidation state (H+1)

O in the -2 oxidation state (O=)

Aminization: Decomposition of proteins and the release of amines and amino acids

OM (proteins) R-NH2 + Energy + CO2

Ammonification:

R-NH2 + HOH NH3 + R-OH + energy

NH4+ + OH-

Nitrification: biological oxidation of ammonia to nitrate

2NH4+ + 3O2 2NO2- + 2H2O + 4H+

2NO2- + O2 2NO3

-

+H2O

Ion/molecule Name Oxidation State

NH3 ammonia -3

NH4+ ammonium -3

N2 diatomic N 0

N2O nitrous oxide +1

NO nitric oxide +2

NO2- nitrite +3

NO3- nitrate +5

H2S hydrogen sulfide -2

SO4= sulfate +6

N: 5 electrons in the outer shell

loses 5 electrons (+5 oxidation state NO3)

gains 3 electrons (-3 oxidation state NH3)

O: 6 electrons in the outer shell

is always being reduced (gains 2 electrons to fill the outer shell)

H: 1 electron in the outer shell

N is losing electrons to O because O is more electronegative

N gains electrons from H because H wants to give up electrons

Hydrogen:

Electron configuration in the ground state is 1s1 (the first electron shell has only one electron in it), as found in H2 gas.

s shell can hold only two electrons, atom is most stable by either gaining another electron or losing the existing one. Gaining an electron by sharing occurs in H2, where each H atom gains an electron from the other resulting in a

pair of electrons being shared. The electron configuration about the atom, where: represents a pair of electrons, and may be shown as

H:H and the bond may be shown as H-H

Hydrogen most commonly exists in ionic form and in combination with other elements where it has lost its single electron. Thus it is present as the H+ ion or brings a + charge to the molecule formed by combining with other elements.

Oxygen:

Ground state of O, having a total of eight electrons is 1s2, 2s2, 2p4.

Both s orbitals are filled, each with two electrons.

The 2p outer or valence orbital capable of holding six electrons, has only four electrons, leaving opportunity to gain two. The common gain of two electrons from some other element results in a valence of -2 for O (O=). The gain of two electrons also occurs in O2 gas, where two pairs of electrons are shared as

O::O and the double bond may be shown as O=O

Nitrogen:

Ground state of N is 1s2, 2s2, 2p3.

Similar to that for oxygen, except there is one less electron in the valence 2p orbital. Hence, the 2p orbital contains three electrons but, has room to accept three electrons to fill the shell. Under normal conditions, electron loss to for N+, N2+ or N3+ or electron gain to form N-, N2-, or N3- should not be expected. Instead, N will normally fill its 2p orbital by sharing electrons with other elements to which it is chemically (covalent) bound. Nitrogen can fill the 2p orbital by forming three covalent bonds with itself as in the very stable gas N2.

Nitrogen Cycle:

•Increased acidity?

Ammonia Volatilization

· Urease activity (organic C) · Air Exchange

· Temperature · N Source and Rate

· CEC (less when high) · Application method

· H buffering capacity of the soil · Crop Residues

· Soil Water Content

NH4+ NH3 + H+

If pH and temperature can be kept low, little potential exists for NH3 volatilization. At pH 7.5, less than 7% of the ammoniacal N is actually in the form of NH3 over the range of temperatures likely for field conditions.

6

7

8

9

10

0 20 40 60 80 100

pH

%

NH 3

4+NH

Equilibrium relationship for ammoniacal N and resultant amount of NH3 and NH4 as affected by pH

for a dilute solution.

H20 H+ + OH-

Chemical EquilibriaA+B AB

Kf = AB/A x B

AB A+B

Kd = A x B/AB

Kf = 1/Kd (relationship between formation and dissociation constants)

Formation constant (Log K°) relating two species is numerically equal to the pH at which the reacting species have equal activities (dilute solutions)

pKa and Log K° are sometimes synonymous

Henderson-Hasselbalch

pH = pKa + log [(base)/(acid)]

when (base) = (acid), pH = pKa

Acidification from N Fertilizers (R.L. Westerman)

1. Assume that the absorbing complex of the soil can be represented by CaX

2. Ca represents various exchangeable bases with which the insoluble anions X are combined in an exchangeable form and that X can only combine with one Ca

3. H2X refers to dibasic acid (e.g., H2SO4)

(NH4)2SO4 -----> NH4+ to the exchange complex, SO4

= combines with the base on the exchange complex replaced by NH4

+

Volatilization losses of N as NH3 preclude the development of H+ ions produced via nitrification and would theoretically reduce the total potential development of acidity.

Losses of N via denitrification leave an alkaline residue (OH-)

Reaction of N fertilizers when applied to soil (Westerman, 1985)

______________________________________________________________________1. Ammonium sulfate

a. (NH4)2SO4 + CaX ----> CaSO4 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2Oc. 2HNO3 + CaX ----> Ca(NO3)2 + H2X

Resultant acidity = 4H+ /mole of (NH4)2SO4

2. Ammonium nitratea. 2NH4NO3 + CaX ----> Ca(NO3)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X + 2H2Oc. 2HNO3 + CaX ----> Ca(NO3)2 + H2X

Resultant acidity = 2H+ /mole of NH4NO3

3. Ureaa. CO(NH2)2 + 2H2O ----> (NH4)2CO3

b. (NH4)2CO3 + CaX ----> (NH4)2X + CaCO3

c. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Od. 2HNO3 +CaX ----> Ca(NO3)2 + H2Xe. H2X + CaCO3 neutralization >CaX + H2O + CO2

Resultant acidity = 2H+ /mole of CO(NH2)2

4. Anhydrous Ammoniaa. 2NH3 +2H2O ----> 2NH4OHb. 2NH4OH + CaX ----> Ca(OH)2 + (NH4)2Xc. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Od. 2HNO3 + CaX ----> Ca(NO3)2 + H2Xe. H2X + Ca(OH)2 neutralization > CaX + 2H2O

Resultant acidity = 1H+/mole of NH3

5. Aqua Ammoniaa. 2NH4ON + CaX ----> Ca(OH)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Oc. 2HNO3 +CaX ----> Ca(NO3)2 + H2Xd. H2X + Ca(OH)2 neutralization > CaX +2H2O

Resultant acidity = 1H+/mole of NH4OH

6. Ammonium Phosphatea. 2NH4H2PO4 + CaX ----> Ca(H2PO4)2 + (NH4)2Xb. (NH4)2X + 4O2 nitrification >2HNO3 + H2X +2H2Oc. 2HNO3 +CaX ----> Ca(NO3)2 + H2X

Resultant acidity = 2H+/mole of NH4H2PO4

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3. nitrogen cycle

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